Question

light bulbs. the power rating of a light bulb (such as a 100 w bulb) is the power it dissipates when connected across a 120 v potential difference. what is the resistance of (a) a 100 w bulb and (b) a 60 w bulb? (c) how much current does each bulb draw in normal use?

Answer 1

P = I * V definition of power

a) P = V^2 / R since = I = V / R

R = V^2 / P = 120^2 / 100 = 144 ohms

b) R = 120^2 / 60 = 240 ohms

c) I = P / V from top equation

I1 = 100 W/ 120 V = .833 amps (P = 120^2 / 144 = 100 W)

I2 = 60 W / 120 V = .5 amps ( P = 120^2 / 240 = 60 W